Demonstration:
Hydrochloric acid is electrolytically decomposed into hydrogen and chlorine gases. The gaseous mixture is then caused to react, converting hydrogen and chlorine back into hydrogen chloride. The reaction proceeds via chain mechanism; a camera flash is used to provide the photon necessary for chain initiation.
Materials:
Assembling the Electrolysis Apparatus:
The bulb of pipet #1 is cut away from the stem, as shown by the dotted line, above. The stem is discarded. A hole is drilled in the base of the bulb, using a #2 cork borer and pipet #2 is inserted through the hole, as shown below:
With the bulbs about 1 mm apart, the stem of pipet #2 is cut as shown by the dotted line, above.
The bulb of pipet #1 is slid up near the cut (but not off the stem of #2), and a push-pin type thumb tack is used to poke holes just at the edge of the bulb shoulders of #2. Pencil leads are inserted through the holes; these will be the electrodes.
The natural tendency of the electrodes to angle in and touch can be arrested by a small rubber band or a bit of tape. (Shaded in figure above.)
Bulb #1 is now a water reservoir. The entire apparatus is clamped in place in an upright position.
Preparation of the Projectile.
The wide end of a cork that will fit snugly into the mouth of the tube is
glued (Elmer's) to the ping-pong ball.
The Launching Pad.
A large cork, drilled to snugly accept the tube, is clamped in a utility clamp (or similar), pointing upward at 30-45º. While I have had no instances of tube breakage, you may wish to
use a plastic "shield" such as the plastic case in which capillary tubes are sold.
The Electrolysis.
Fill bulb #2 with 10-11 M HCl (prepared by adding a little water to concentrated acid to reduce off-gassing). Invert a 10 x 100-mm test tube (or similar size) that has been filled with water over the upright stem of bulb #2. Connect the wires from a 9-volt battery to begin electrolysis. Hydrogen and chlorine (and oxygen) gases will begin to fill the tube. When gas is bubbling out of the tube, quickly refill it again (this ensures that a good concentration of chlorine will be obtained, since it is rather soluble in water). When the tube is again filled with the gas mixture, proceed to the next stage.
The Rocket.
Quickly lift the tube clear of the water reservoir, allowing residual solution to drain, and plug the tube with the cork which is attached to the ping-pong ball. The tube is now inserted into a larger cork which has been drilled to accept it.
The large cork is used to clamp the tube in place at a 45º angle, as shown in the figure above. If desired, a large plastic tube such as the ones that come with melting point capillaries may be used as a sort of safety shield (see figure), but I have not found that necessary, nor do I expect to, since I am using a substantial tube.
With the tube clamped in place and at least 2-3 cm of the tube exposed above the large cork (more is better), the flash of the camera is placed very close to the glass wall of the tube and triggered. The flash photo-ignites the hydrogen-chlorine mixture, expelling the small cork and ping pong ball.
As time and creativity allow, you might choose to decorate the ping pong ball.
Flinn Scientific Foundation Workshops 1 Copyright © John G. Little
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